I would say the pike, the record for which sits at around 46lb. The record for carp is around 66lb- it all depends whether you count this as a native or not- was introduced by the romans. Likewise Wells Catfish officially is around 62lb but there are reports of a monster over 90lb. But introduced from Europe.
And what about Atlantic Salmon- the record is around 64lb- but do you count it as a freshwater fish or game fish? The record was caught in freshwater- but as salmon spend most of their time in the sea do you count that?